This invention relates to portable power tools and in particular, to a motor and gearbox for a portable power tool. It has special relevance to portable hand held drills and it is in this context that the invention will be explained.
Portable hand held tools such as drills are driven by electric motors, either battery or mains powered. To increase the output torque of the drill, the motor drives the chuck through a speed reduction gear train. To keep the drill compact and for maximum drilling force, epicyclic gearboxes are preferred.
Presently, the drill manufacturer receives a motor and a separate gearbox complete with an output shaft. The motor and gearbox are connected together using a special adapter plate and then the combined motor and gearbox are assembled into the casing of the drill. While this approach works well in aligning the motor to the gearbox, there is little room for reducing the overall length of the drill as desired by current market trends.
Japanese Patent Application JP 8-210455 discloses a stepper motor/gearbox combination in which the motor end cap and gearbox cover are combined as a single item with the gearbox end of the motor shaft being supported directly by the planet gears of the gearbox. This provides an axially compact motor/gearbox combination. However, there are concerns about the stability of the motor, especially under high torque/high load applications experienced, for example, in a power tool application such as a drill. For motors with very small air gaps such as DC motors used in hand held portable cordless drills, the radial movement of the motor shaft allowed by the meshing of the planet gears with the sun gear of the shaft may affect the performance of the motor.